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Radioactive Decay
Overview The Basics Here's a chart explaining the basics of radioactive decay. Image taken from http://spurr.pls.uni.edu/ctst/inspiration.html kGeneral Rules Over the years, scientists have managed to determine some general rules to predict how likely it is that an atom will undergo radioactive decay. An element will undergo decay if: #It has more than 83 protons. #The ratio of protons to neutrons is very close. For example, Carbon, having 6 protons and 6 neutrons (therefore having a 1:1 ratio of protons and neutrons) is a very stable atom. On the other hand, Bismuth, an unstable atom, has 126 neutrons and 83 protons, giving it a 1.52:1 ratio. #Nuclides containing certain numbers of protons or neutrons (2, 8, 20, 50, 82, and 126) are more stable than others. #Nuclides with even numbers of both protons and neutrons and more stable than those with odd numbers of both. The History The concept of radioactive decay was first discovered in 1896 by Henri Becquerel as he was working the element uranium compounds. In his first experiment, he placed the uranium on top of photographic film wrapped in dark paper and placed the crystals in the sunlight. The film darkened in response to some type of energy, which Bequerel assumed to be the prescence of the sun. However, Becquerel's second experiment revealed something much more interesting. After placing the uranium in an enclosed spot with no sunlight at all, Becquerel expected that the paper would be clear and free of dark spots. Much to his surprise, the paper was darkened as if it had been exposed to the sun. Becquerel concluded that the uranium must be emitting some sort of high energy. He had discovered radioactivity. Later, Becquerel demonstrated that the radiation emitted by uranium was similar to X rays but, unlike X rays, could be deflected by a magnetic field and therefore must consist of charged particles. For his discovery of radioactivity, Becquerel was awarded the 1903 Nobel Prize for physics. With the help of Madame Curie and her husband Pierre, Becquerel was able to isolate two other radioactive substances: polonium and radium. In fact, Pierre and Marie Cuerie found that radium was over 300,000 times stronger than uranium! Following this discovery, in 1899 the scientist Ernest Rutherford (1871-1937) discovered alpha, beta, and gamma radiation. A substance, by emitting any of these particles, changes itself by a process called "transmutation". Alpha Decay An alpha particle, represented by the notation He2+, consists of two protons and two neutrons, meaning that it must come from the nucleus of an atom. The loss of these charges causes the nucleus to transmute: the mass number of the decaying nucleus is reduced by 4 while the atomic number is reduced by two. http://education.jlab.org/glossary/alphadecay.gif Image taken from http://education.jlab.org/glossary/alphadecay.html For example, in the decay of gold: Gold, which has an atomic number of 79, becomes an element with an atomic number of 77, and changes from an atomic weight of 185 to an atomic weight of 181. Using a Periodic Table of elements, the element fitting those requirements is Iridium. Alpha particles are emitted from the nucleus atom at 10 percent the speed of light. Because the penetrating power of alpha particles is so low, it can't even go through a sheet of paper! However, the alpha particle will do severe damage to the cells if taken into the body, such as through inhalation. Beta Decay Beta decay is caused by electrons and antineutrinos being emitted by the nucleus. For this to happen, a neutron within the nucleus must be changed into a proton. In beta decay, the atomic number increases by 1 but the mass remains unchanged. Following the Law of Conservation of Charge, another negative charge must be created. Consequently, charge is conserved. In the beta decay of Helium, a beta particle (an electron) is emitted and the element becomes Lithium. Gamma Radiation When exposed to gamma radiation a nucleus changes from a higher energy state to a lower energy state through the emission of photons. Because of this, gamma rays move at the speed of light. However, neither mass nor atomic number is changed when the nucleus emits gamma rays. Gamma radiation usually occurs with either beta or alpha decay. Gamma rays easily penetrate the body, but can be stopped by lead or concrete. Gamma rays are especially harmful because, even at low levels, there are known to cause problems within the human body. For this reason, doctors try to use x-rays with discretion. Half-Life and Carbon Dating Believe it or not, radioactive decay has become quite a useful tool in modern science. Remember when I said that atoms that are have equal ratios of protons and neutrons, like Carbon, tend to be more stable than those that don't? In fact, Carbon is so stable that it is used in a modern process called "carbon dating". Scientists are able to use Carbon to compare with other elements, determining the age of various materials, such as fossils. Scientists are able to do this using a special property of all radioactive materials called "half-life". A half-life is the amount of time it takes for half of the reactant to be converted into its product. This half-life can also be calculated using the equation: http://library.thinkquest.org/10429/media/nuclear/halflife/halfequ.gif Image from http://library.thinkquest.org/10429/high/nuclear/nubody.htm Where: * AE is the amount of substance left * A0 is the original amount of substance * t is the elasped time * t1/2 is the half-life of the substance As well as being used for scientific purposes, Carbon dating can also be used for health reasons. When there are accidents at nuclear power plants, it is imperative for the people to know exactly when environments are safe to return to. Carbon dating can accurately tell us the minimun amount of time needed to stay clear until the radioactive substance is past the point of being dangerous to humans. Here is a chart showing some half-lives of radioactive substances: http://www.agiweb.org/news/evolution/figures/dating3.jpg Image from: http://www.agiweb.org/news/evolution/dating3.html Practice Problems Fill in the spaces with the appropriate elements or particles. Click here for the answers!http://dbhs.wvusd.k12.ca.us/webdocs/Radioactivity/Alpha-Exercises-Ans.html Resources and References Resources #http://dbhs.wvusd.k12.ca.us/webdocs/Radioactivity/Writing-Alpha-Beta.html -- For more practice problems and solutions. #http://www.aip.org/history/curie/resbr1.htm -- More information on Becquerel and the scientists who helped him! #http://www.1728.org/halflife.htm -- A useful calculator that will compute the half-life of any element. #http://www.dayah.com/periodic/ -- The periodic table! All the information you could ever want about any element. References #Zitzewitz, Paul W. Physics: Principles and Problems. New York: Glencoe/McGraw-Hill, 1999. #Guch, Ian. The Complete Idiot's Guide to Chemistry. USA: Alpha Books, 2003. -- Like the title says, this book is a clear and concise way to learn about all things concerning chemistry. Everything is in simple terms, but it covers a wide variety of topics and provides many practice problems. #http://physics.bu.edu/py106/notes/RadioactiveDecay.html -- Basic overview of radioactive decay.